B and T lymphocytes, the primary cells of the adaptive immune system, provide a major line of defense against a myriad of foreign molecules by virtue of antigen-specific receptors. The vast diversity amongst the antigen receptors genes is generated through V(D)J recombination, a process in which individual V, D, and J gene segments are rearranged. Defects in V(D)J recombination lead to combined immunodeficiencies in human patients and can also result in lymphoid malignancies. One factor that plays a critical role during V(D)J recombination is the non-homologous end joining (NHEJ) DNA repair factor, Artemis. Artemis was initially discovered as the gene inactivated in a human radiosensitive severe combined immunodeficiency syndrome. Hypomorphic Artemis alleles have also been identified in patients and are associated with combined immunodeficiencies of varying severity. Artemis plays a vital role as a DNA nuclease that processes broken ends during V(D)J recombination as well as general double strand break repair prior to ligation. Its intrinsic exo- and endonucleolytic activities are modulated by interaction with the DNA-dependent protein kinase catalytic subunit NHEJ factor. However, the precise mechanisms by which Artemis activities are regulated in vivo are not well understood. The major goals of this proposal are to gain a better understanding of the mechanisms and regulation of DNA end processing during V(D)J rearrangements and general DNA repair. Three specific aims are proposed. Aim 1 is to define the molecular mechanisms involved in activating and regulating Artemis endonucleolytic activities. Aim 2 is to elucidate the molecular mechanisms underlying tumorigenesis caused by a hypomorphic, Artemis disease allele, P70, using the mouse as a model system. This premature translation termination mutation leads to partial immunodeficiency and predisposition to lymphoid malignancy in human patients. In addition, the genetic interactions between Artemis, ATM and Mre11 will be examined. Aim 3 will capitalize on the previously developed Artemis-P70 mouse model, which provides a valuable in vivo system to define the impact of therapeutics that allow read-through of nonsense mutations. The impact of this emerging class of drugs on the DNA repair, genome instability, V(D)J recombination, and lymphocyte development phenotypes in mutant cells and mice will be determined. Together, these studies will provide important insights into the molecular events that occur during V(D)J coding end joining and general DSB repair, elucidate the impact of aberrant V(D)J end processing on lymphocyte development and tumor predisposition, and potentially identify novel therapeutics for primary immunodeficiencies caused by nonsense mutations.